Final answer:
To find out how long it will take for R4000 to earn R840 at 3% simple interest per annum, the formula T = I / (P * R) is used, resulting in 7 years.
Step-by-step explanation:
The student's question involves calculating the time it takes for an investment to earn a certain amount of interest at a given simple interest rate. To determine the length of time (T) it will take for an initial investment of R4000 to earn R840 in interest at an annual simple interest rate of 3%, we can use the simple interest formula which is I = PRT, where I is the interest earned, P is the principal amount, R is the rate of interest, and T is the time in years.
Firstly, we will rearrange the formula to solve for T: T = I / (P * R). Plugging the values into the formula, we have T = R840 / (R4000 * 0.03). Simplifying this, T = R840 / R120 = 7 years.
Therefore, it will take 7 years for the investment of R4000 at a 3% per annum simple interest rate to earn R840 in interest.